A control circuit of Telefunken is already known, which is designed as an integrated circuit and is identified by U2008B. This integrated circuit is supplied by the rectified d.c. current supply. The ground connector is directly connected with the second net connector. If a potential separation between the load current circuit and control current circuit of the triac is required, a transformer element such as an optical coupler or an ignition transformer is required. Not only is this additional element relatively expensive, but it also represents an additional failure risk.